Method for sweeping charged particles out of an isochronous cyclotron, and device therefor

ABSTRACT

A method for extracting a charged particle beam out of an isochronous cyclotron (1) comprising an electromagnet forming a magnetic circuit that includes at least a number of sectors (3, 3&#39;) known as &#34;hills&#34; where the air-gap is reduced, and separated by sector-shaped spaces (4) known as &#34;valleys&#34; where the air-gap is larger. According to the extraction method, the particle beam is extracted without using an extraction device as the magnetic field has a special arrangement produced by designing the electromagnet air-gap at the &#34;hills&#34; (3, 3&#39;) of the isochronous cyclotron in such a way that the aspect ratio between the electromagnet air-gap at the &#34;hills&#34; in the region of the maximum radius, and the radius gain per turn of the particles accelerated by the cyclotron at said radius is less than 20.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the national stage of International Application No.PCT/BE96/00101 filed Sep. 25, 1996.

SUBJECT OF THE INVENTION

The present invention relates to a method of extracting chargedparticles from an isochronous cyclotron in which the particle beam isfocused by sectors.

The present invention also relates to the said isochronous cyclotronwhich applies this method of extracting charged particles.

The present invention relates both to compact isochronous cyclotrons andto cyclotrons focused by sectors. Similarly, the present inventionrelates to isochronous cyclotrons referred to as superconducting ornon-superconducting.

Prior art

Cyclotrons are particle accelerators used, in particular, for theproduction of radioactive isotopes. These cyclotrons are usuallycomposed of two distinct main assemblies, consisting on the one hand ofthe electromagnet and on the other hand of the radiofrequency resonator.

The electromagnet guides the charged particles on a path approximatelyrepresenting a spiral whose radius increases around the acceleration. Inmodern cyclotrons of the isochronous type, the electromagnet poles aredivided into sectors which alternately have a reduced air gap and alarger air gap. The azimuthal variation in the magnetic field whichresults therefrom has the effect of focusing the beam vertically andhorizontally during the acceleration.

Among isochronous cyclotrons, distinction should be made betweencyclotrons of the compact type, which are excited by at least one maincircular coil, and cyclotrons referred to as having separate sectors, inwhich the magnetic structure is divided into fully self-containedseparate units.

The second assembly consists of the accelerating electrodes, frequentlyreferred to as "dees" for historical reasons. An alternating voltage ofseveral tens of kilovolts is thus applied to the electrodes at thefrequency of rotation of the particles in the magnet, or alternately ata frequency which is an exact multiple of the frequency of rotation ofthe particles in the magnet. This has the effect of accelerating theparticles of the beam circuiting in the cyclotron.

For a number of applications which use a cyclotron, it is necessary toextract the beam of accelerated particles from the cyclotron and guideit to a target where it is intended to be used. This beam extractionoperation is considered by the person skilled in the art to be the mostdifficult step in the production of a beam of accelerated particlesusing a cyclotron. This operation consists in bringing the beam from thepart of the magnetic field where it is accelerated to the point wherethe magnetic field is no longer capable of holding the beam. In thiscase, the beam is free to escape from the influence of the field and isextracted from the cyclotron.

In the case of cyclotrons which accelerate positively charged particles,it is known to use an electrostatic deflector, the purpose of which isto pull the particles out of the magnetic field in the manner of anextraction device. In order to obtain an effect of this type, it isnecessary for an electrode, which is referred to as the septum and willintersect a fraction of the particles, to be interposed on the path ofthese particles. For this reason, the extraction efficiency isrelatively limited, and the loss of particles in the septum willcontribute, in particular, to making the cyclotron highly radioactive.

It is also known to extract negatively charged particles by convertingthe negative ions into positive ions by passing them through a sheetwhose function is to strip the electrons from the negative ions. Thistechnique makes it possible to obtain extraction efficiencies close to100% and also makes it possible to use a device which is must lesscomplex than the one described above. Nevertheless, for its part, theacceleration of the negative particles presents major difficulties. Themain drawback resides in the fact that the negative ions are fragile,and are therefore readily dissociated by residual gas molecules orexcessive magnetic fields which are present in the cyclotron and throughwhich the ions pass at high energy. The transmission of the beam in theaccelerator is therefore limited, which also contributes to itsactivation.

On the other hand, cyclotrons which accelerate positive particles makeit possible to produce greater beam currents and make the system morereliable, while permitting a significant reduction in the size andweight of the machine.

A technique is also known, from The Review of Scientist Instruments, 27(1956), No. 7 and from Nuclear Instruments and Methods 18, 19 (1962),pp. 41-45 by J. Reginald Richardson, according to which method it wouldhave been possible to extract the particle beam from the cyclotronwithout using an extraction device. The conditions required to obtainthis auto-extraction are particular conditions relating to resonance ofthe motion of the particles in the magnetic field.

Nevertheless, this described method is particularly difficult toimplement, and would have given a beam whose optical qualities were sopoor that it has never been applied in practice.

U.S. Pat. No. 0,324,379 relates to a device of the cyclotron type whichis intended to accelerate particles and has magnetic means that areessentially independent of the azimuthal angle. This means that thecyclotron is a non-isochronous one. It should furthermore be noted thatthe cyclotron which is described has beam extraction means which consistof "regenerators" and "compressors" which, by perturbing the magneticfield, make it possible to extract the particle beam.

WO-93/10651 in the name of the Applicant Company describes a compactisochronous cyclotron having an air gap located between two hills, ofessentially elliptical shape and tending to close on itself completelyat the radial end of the hills on the median plane. The device describedin this document also comprises conventional beam extraction meanswhich, in the present case, consist of an electrostatic deflector.

OBJECTS OF THE PRESENT INVENTION

One object of the present invention is to provide a method of extractingcharged particles from an isochronous cyclotron while avoiding the useof extraction devices such as the ones described above.

An additional object of the present invention is therefore to provide anisochronous cyclotron which is of simpler and more economical designthan those used conventionally.

A further object of the invention is to increase the particle beamextraction efficiency, in particular in the case of extracting positiveparticles.

MAIN CHARACTERISTIC ELEMENTS OF THE PRESENT INVENTION

The present invention relates to a method of extracting chargedparticles from an isochronous cyclotron having an electromagnetconstituting the magnetic circuit which includes a certain number ofpairs of sectors, referred to as "hills", where the air gap is reduced,these being separated by spaces in the form of sectors, referred to as"valleys", where the air gap is of larger size; this method beingcharacterized in that an isochronous cyclotron is produced with a magnetair gap between the hills whose dimensions are chosen in such a way thatthe minimum value of this air gap in the vicinity of the maximum radiusbetween the hills is less than twenty times the gain in radius percircuit of the particles accelerated by the cyclotron at this radius.

According to this particular configuration, it will be observed that theions can be extracted from the influence of the magnetic field withoutthe assistance of any extraction device.

It should be noted that, in the case of prior art isochronouscyclotrons, the air gap of the magnet is in general between 5 and 20 cm,while the gain in radius per circuit is about 1 mm. In this case, theratio of the air gap to the gain in radius per circuit is greater than50.

It will be observed that, when the main characteristic of the presentinvention is applied, the magnetic field decreases very abruptly in thevicinity of the limit of the pole of the magnet, so that theauto-extraction point is reached before the phase shift of the particleswith respect to the accelerating voltage reaches 90 degrees. In thisway, the particles leave the magnetic field automatically without theintervention of any extraction device.

According to a particularly preferred embodiment of the presentinvention, it may be envisaged to design an air gap having an ellipticalprofile which tends to close on itself at the radial end of the hills,as described in Patent WO93/10651.

According to a preferred embodiment of the present invention, theextraction of the particles is concentrated on one sector by virtue ofan asymmetry given deliberately to the shape or to the magnetic field ofthe said sector.

According to another preferred embodiment of the present invention, theangle of one of the sectors is reduced at the pole radius in order tomake it possible to shift the orbits and thus to obtain the extractionof the entire beam on this side so as, for example, to make it possibleto irradiate a target of large volume.

According to another preferred embodiment of the present invention, aparticular distribution of the particle beam is produced so assimultaneously to irradiate a plurality of targets mounted side by sideon the path of the beam.

The present invention can advantageously be used for proton therapy orthe production of radioisotopes, and more particularly radioisotopesintended for positron emission tomography (PET).

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 and 2 represent magnetic profiles of a prior art isochronouscyclotron and of an isochronous cyclotron using the extraction methodaccording to the present invention.

FIG. 3 schematically represents an exploded view of the main elementsconstituting an isochronous cyclotron.

FIG. 4 represents a cross-section of an isochronous cyclotron.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The profile of the magnetic field in an isochronous cyclotron is suchthat the frequency of rotation of the particles should be constant andindependent of their energy. In order to compensate for the increase inthe relativistic mass of the particles, the magnetic field shouldtherefore increase with the radius in order to ensure this isochronismcondition. To describe this relationship, the field index is defined bythe following equation: ##EQU1## in which dB/B and dR/R are respectivelythe relative variations in the magnetic field and in the radius atradius R.

It should be noted that it is impossible to maintain the isochronismcondition in the vicinity of the maximum radius of the pole. The reasonfor this is that, at this moment, the field ceases to increase with theradius. It has reached a maximum and then starts to decrease more andmore rapidly.

FIG. 1 illustrates the variation in the field as a function of theradius in a conventional isochronous cyclotron. An increasing phaseshift is set up between the frequency of rotation of the particles andthe resonant frequency of the accelerating electrodes. When this phaseshift reaches 90 degrees, the particles cease to be accelerated andcannot exceed this radius.

FIG. 2 illustrates the variation in the field as a function of theradius in an isochronous cyclotron using the extraction method accordingto the present invention. By accurately choosing the dimensions of theair gap of the magnet between the hills in such a way that it is reducedto a value of less than twenty times the gain in radius per circuit, amagnetic field profile as represented in FIG. 2 is observed.

In this case, the magnetic field decreases very abruptly in the vicinityof the limit of the pole of the magnet, so that the auto-extractionpoint defined by the field index n=-1 is reached before the phase shiftof the particles with respect to the accelerating voltage reaches 90degrees.

From this moment on, the particles automatically leave the magneticfield without the intervention of any extractor device.

An isochronous cyclotron as used in the method of extracting chargedparticles according to the present invention is representedschematically in FIGS. 3 and 4. This cyclotron is a compact isochronouscyclotron intended for the acceleration of positive particles, and moreparticularly protons.

The magnetic structure 1 of the cyclotron is composed of a certainnumber of elements 2, 3, 4 and 5 made of a ferromagnetic material andcoils 6 preferably made of a conductive or superconductive material. Inconventional fashion, the ferromagnetic structure comprises:

two base plates 2 and 2', referred to as yokes,

at least three upper sectors 3, referred to as hills, and an equalnumber of lower sectors 3', which are located symmetrically relative toa plane of symmetry 10, referred to as the median plane, with respect tothe upper sectors 3, and which are separated by a small air gap 8,

between two successive hills there is a space where the dimension of theair gap is greater, and this is referred to as a valley 4,

at least one flux return 5 rigidly joining the lower yoke 2 to the upperyoke 2'.

The coils 6 are of essentially circular shape and are located in theannular space left between the sectors 3 or 3' and the flux returns 5.

The central channel is intended to accommodate at least a part of thesource of particles 7 to be accelerated. These particles are injected atthe centre of the apparatus by means which are known per se.

For an isochronous cyclotron accelerating a proton beam to an energy of11 MeV, the magnet is designed, according to the invention, with an airgap of 10 mm for a magnetic field of 2 teslas on the magnetic sectors 3and 3'. The accelerating voltage is 80 kilovolts, so as to obtain a gainin radius of 1.5 mm at the maximum radius.

This unusual choice of parameters makes it possible, at the radialextremity of the hills, to observe an extremely rapid decrease in theexternal induction, which makes it possible to auto-extract the particlebeam before the acceleration limit, and this is more particularlyrepresented in FIG. 2.

According to a first preferred embodiment, the angle of one of thesectors is reduced at the pole radius so as to make it possible to shiftthe orbits and obtain extraction of the entire beam on this side (seeFIG. 4).

The extracted particle beam is then axially focused and radiallydefocused.

According to another preferred embodiment, this beam profile is used forthe simultaneous irradiation of four targets located between the twocoils 6 mounted side by side on the path of the beam.

I claim:
 1. Method of extracting a beam of charged particles from anisochronous cyclotron (1) having an electromagnet constituting themagnetic circuit which includes at least a certain number of sectors (3,3'), referred to as "hills", where the air gap is reduced, these beingseparated by spaces in the form of sectors (4), referred to as"valleys", where the air gap is of larger size, the extraction methodbeing characterized in that the particle beam is extracted by aparticular arrangement of the magnetic field, without resorting to anextraction device, this arrangement being obtained by designing the airgap of the magnet at the hills (3, 3') of the isochronous cyclotron insuch a way that the ratio of the dimension of the air gap of the magnetat the hills in the vicinity of-the maximum radius to the gain in radiusper circuit of the particles accelerated by the cyclotron at this radiusis less than
 20. 2. Isochronous cyclotron in which the particle beam isfocused by sectors and which has an electromagnet constituting themagnetic circuit which includes at least a certain number of sectors (3,3'), referred to as "hills", where the air gap is reduced, these beingseparated by spaces in the form of sectors (4), referred to as"valleys", where the air gap is of larger size, characterized in thatthe air gap of the magnet at the hills (3, 3') is designed in such a waythat the ratio of the dimension of the air gap of the magnet at thehills in the vicinity of the maximum radius to the gain in radius percircuit of the particles accelerated by the cyclotron at this radius isless than
 20. 3. Isochronous cyclotron according to claim 2,characterized in that the profile of the air gap of the magnet at thehills is an elliptical profile tending to close on itself at the radialend of the hills.
 4. Cyclotron according to claim 2, characterized inthat at least one sector has a shape or a magnetic field that isasymmetric with respect to the other sectors.
 5. Cyclotron accordingclaim 2, characterized in that the angle of one of the sectors isreduced at the pole radius.
 6. Cyclotron according to claim 2,characterized in that a particular distribution of the particle beam isproduced so as simultaneously to irradiate a plurality of targetsmounted side by side on the path of the beam.